J. Phys. Chem. 1989, 93, 1244-1248
1244
Conformations of Naphthalene Dimer Cation Radicals Studied by Laser Photolysis Akira Tsuchida, Yoshinobu Tsujii, Shinzaburo Ito, Masahide Yamamoto,* Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Yoshida, Sakyo- ku, Kyoto 606, Japan
and Yoshio Wada Department of Photographic Engineering, Kyoto Institute of Technology, Matsugasaki. Sakyo-ku, Kyoto 606, Japan (Received: March 23, 1988; In Final Form: July 29, 1988)
To study the conformations of naphthalene dimer cation radicals in solution their transient absorption spectra were obtained by nanosecond laser photolysis measurements. The transient absorption spectrum for an intramolecular bichromophoric compound 1,3-di(2-naphthyl)propane (1,3-22Np) gave two different bands of dimer cation radicals at around 580 and 660 nm, whereas that for I-( l-naphthyl)-3-(2-naphthyl)propane (1,3-12Np) gave only the band of dimer cation radical at ca. 580 nm. 1,3-22Npcan have two overlapping conformationsof naphthyl chromophores (eclipsed and staggered), while 1,3-12Np can have only a staggered conformation. Therefore, the 580- and 660-nm absorption bands were ascribed to the staggered and eclipsed dimer cation radicals, respectively. The transient absorption spectra of the meso and racemic diastereoisomers of 2,4-di(2-naphthy1)pentane gave two absorption bands of the eclipsed and staggered dimer cation radicals.
Introduction Photophysical and photochemical properties of many intramolecular bichromophoric compounds have been extensively studied.]-]* When two aromatic chromophores are linked by a ( I ) For reviews see: (a) De Schryver, F. C.; Collart, P.; Vandendriessche, J.; Goedeweeck, R.; Swinnen, A.; Van der Auweraer, M. Acc. Chem. Res. 1987, 20, 159. (b) Semerak, S. N.; Frank, C. W. Adu. Polym. Sci. 1984,54, 31. (2) Review books: (a) Guillet, J. Polymer Photophysics and Photochemistry; Cambridge University Press: London, 1985. (b) Roberts, A. J.; Soutar, I. In Polymer Photophysics; Phillips, D., Ed.; Chapman and Hall: London, 1985; Chapter 5. (c) Winnik, M. Photophysical and Photochemical Tools in Polymer Science; Reidel: Dordrecht, 1986. (3) Hirayama, F. J. Chem. Phys. 1965, 42, 3163. (4) For phenyl, carbazolyl, and pyrenyl compounds, see: (a) De Schryver, F. C.; Moens, L.; Van der Auweraer, M.; Boens, N.; Monnerie, L.; Bokobza, L. Macromolecules 1982, 15,64. (b) De Schryver, F. C.; Vandendriessche, J.; Toppet, S.; Demeyer, K.; Boens, N. Macromolecules 1982, 15,406. (c) Collart, P.; Demeyer, K.; Toppet, S.; De Schryver, F. C. Macromolecules 1983, 16, 1390. (d) Vandendriessche, J.; Palmans, P.; Toppet, S.; Boens, N.; De Schryver, F. C.; Masuhara, H. J. Am. Chem. SOC.1984, 106, 8057. (e) Collart, P.; Toppet, S.; Zhou, Q. F.; Boens, N.; De Schryver. F. C. Macromolecules 1985, 18, 1026. (f) Goedeweeck, R.; Van der Auweraer, M.; De Schryver, F. C. J. Am. Chem. SOC.1985, 107, 2334. (g) Collart, P.; Toppet, S.; De Schryver, F. C. Macromolecules 1987, 20, 1266. (5) Naphthyl: (a) De Schryver, F. C.; Demeyer, K.; Van der Auweraer, M.; Quanten, E. Ann. N. Y. Acad. Sci. 1981, 366, 93. (b) Todesco, R.; Gelan, J.; Martens, H.; Put, J.; De Schryver, F. C. J. Am. Chem. SOC.1981, 103, 7304. (c) De Schryver, F. C.; Demeyer, K.; Toppet, S. Macromolecules 1983, 16, 89. (d) Pratte, J. F.; Webber, S. E.; De Schryver, F. C. Macromolecules 1985, 18, 1284. (e) Vandendriessche, J.; Collart, P.; De Schryver, F. C.; Zhou, Q. F.; Xu, H. J. Macromolecules 1985, 18, 2321. (6) Ionic states: (a) Masuhara, H.; Vandendriessche, J.; Demeyer, K.; Boens, N.; De Schryver, F. C. Macromolecules 1982, 15, 1471. (b) Masuhara, H.; Tamai, N.; Mataga, N.; De Schryver, F. C.; Vandendriessche, J. J. Am. Chem. SOC.1983, 105, 7256. (c) Masuhara, H.; Tanaka, J. A.; Mataga, N.; De Schryver, F. C.; Collart, P. Polym. J. 1983, 15, 915. (d) Masuhara. H.: Yamamoto, K.: Tamai. N.; Inoue. K.; Mataga, N. J. Phvs. Chem. 1984,88, 3971. (7) Pyrenyl: (a) Zachariasse, K. A,; Kiihnle, W. Z . Phys. Chem. (Wiesbadenl 1976. 101. 267. (bl Zachariasse, K. A.: Duveneck, G.; Busse. R. J . Am. Chem. SOC.1984, 106, 1045. (c) Zachariasse, K. A,; Duveneck, G.; Kiihnle, W. Chem. Phys. Lett. 1985, 113, 337. (d) Zachariasse, K. A,; Duveneck, G.; Kiihnle, W.; Reynders, P.; Striker, G. Chem. Phys. Lett. 1987, 133, 390. (e) Reynders, P.; Dreeskamp, H.; Kiihnle, W.; Zachariasse, K. A. J. Phys. Chem. 1987, 91. 3982. (8) Carbazolyl: (a) Klopffer, W. J. Chem. Phys. 1969, 50, 2337. (b) Johnson, G. E. J. Chem. Phys. 1975, 62, 4697. ( c ) Itaya, A,; Okamoto, K.: Kusabayashi, S. Bull. Chem. SOC.Jpn. 1976, 49, 2082. (d) Hoyle, C. E.; Nemzek, T. L.; Mar, A.; Guillet, J. E. Macromolecules 1978, 11, 429. (e) Ghiggino, K. P.; Wright, R. D.; Phillips, D. Eur. Polym. J. 1978, 14, 567. (9) Naphthyl: (a) Ito, S.; Yamamoto, M.; Nishijima, Y. Rep. Prog. Polym. Phys. Jpn. 1979, 22, 453. (b) Ito, S.; Yamamoto, M.; Nishijima, Y. Bull. Chem. SOC.Jpn. 1981, 54, 35. (c) Ito, S.; Yarnarnoto, M.; Nishijirna, Y. Bull. Chem. SOC.Jpn. 1982, 55, 363. (d) Ito, S.; Yamamoto, M.: Nishijima, Y. Bull. Chem. SOC.Jpn. 1984, 57, 3295.
C, methylene chain, a sandwich conformation is favorable., In this conformation, where the chromophore interaction becomes maximum, the intramolecular excimer and dimer ion radical formation are readily observed.’-* Recently, it has been made clear that 1,3-diaryl-substituted propanes as a model compound of a polymer do not reflect the chromophore interaction of dyads in the polymers, and hence 2,4-diaryl-substituted pentanes have been used as good model In the case of the diastereoisomers of 2,4-di(Ncarbazolyl)pentane, two different excimer formations were obs e r ~ e d . ~ -The ~ + ’meso ~ isomer shows “normal” or “sandwich” excimer emission, where two carbazolyl chromophores show a total overlap of TT conformation, while the racemic isomer shows “high-energy” or “second“ excimer emission, where the chromophores have only a partial overlap in the TT conformation. The preferable formation of the sandwich excimer from the meso isomer and that of the second excimer from the racemic isomer are explained by the starting ground-state conformations of diastereoisomers! In the case of naphthyl or 1-pyrenyl disubstituted pentanes, whose chromophores are substituted at unsymmetrical positions, two types of excimers also have been observed in the sandwich con for ma ti or^.^.^,^,^^ In this report, the words sandwich and second conformation are used in analogy with the conformation of the carbazole chromophore, 2,4-di(N-carbazolyl)pentane. Therefore, sandwich conformation has “eclipsed” (total overlap) and “staggered” (partial overlap) conformations. The eclipsed and staggered excimers are formed in the TT conformation of the meso isomer and the TG/GT conformation of racemic isomer. The ionic states of these bichromophoric compounds6J2have been studied mainly thfough both exterplexI3 emission and transient absorption, and the overlapping structure of chromophores corresponding to each excimer state was proposed. Among aromatic compounds, the naphthalene chromophore has been extensively s t ~ d i e d . ~ ~ J Recently, ~-’~ h i e et al.12 studied the (IO) (a) Goldenberg, M.; Emert, J.; Morawetz, H. J. Am. Chem. SOC. 1978, 100, 7171. (b) Evers, F.; Kobs, K.; Memming, R.; Terrell, D. R. J . Am. Chem. SOC.1983,105, 5988. (c) Todesco, R. V.; Basheer, R. A,; Kamat, P. V. Macromolecules 1986, 19, 2390. (11) (a) Mimura, T.; Itoh, M. J . Am. Chem. Soc. 1976, 98, 1095. (b) Itagaki, H.; Obukata, N.; Okamoto, A.; Horie, K.; Mita, I. J . Am. Chem. Soc. 1982, 104, 4469. (12) (a) hie, S.; Horii, H.; Irk, M. Macromolecules 1980, 13, 1355. (b) hie, S.; Irk, M. Macromolecules 1986, 19, 2182. (13) Beens, H.; Weller, A. Chem. Phys. Lett. 1968, 2, 140. (14) Hamill, W. H. Radical Ions; Kaiser, E. T., Kevan, L., Eds.; Interscience: New York, 1968. ( 1 5) (a) Badger, B.; Brocklehurst, B. Trans. Faraday SOC.1969,65,2588. (b) Badger, B.; Brocklehurst, B. Trans. Faraday SOC.1969, 65, 2939. (c) Badger, B.; Brocklehurst, B.; Russell, R. D. Chem. Phys. Lett. 1967, 1 , 122.
0022-3654/89/2093-1244.$01.50/00 1989 American Chemical Society
Conformations of Naphthalene Dimer Cation Radicals
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1245
CH3 NP
ENP
0.5
n = 3 : 1,3-22Np n = 5 : 1.5-22Np n = 12 : 1.12-22Np
0
1.3-12Np
703
Q,
2
CH3
(bl
1.c
0
rccemi meso
2,L-tNp 2.L-mNp
Figure 1. Molecular structures and abbreviations for naphthalene derivatives used in this study. formation of the dimer cation radicals of 1,3-, 1,6-, and 1,12di(2-naphthy1)alkanes by pulse radiolysis measurements. From the analysis of the transient absorption spectra, they proposed the possibility of the eclipsed and staggered forms of the dimer cation radicals. In the present paper, conformations of inter- and intramolecular naphthalene dimer cation radicals were studied by nanosecond laser photolysis measurements using the compounds shown in Figure 1. The analysis of absorption spectra of 1,3di(2-naphthy1)propane (1,3-22Np) and 1-( l-naphthyl)-3-(2naphthy1)propane (1,3-12Np) enables us to assign the transient absorption bands to the eclipsed and staggered dimer cations. The transient absorption spectra of two diastereoisomers of rac- and meso-2,4-di(2-naphthyl)pentaneswere also measured and two types of dimer cation radicals were observed.
Experimental Section Chemicals. Naphthalene (Np, Wako Pure Chem. Ind.) was purified by recrystallization from ether three times. 2-Ethylnaphthalene (ENp, Tokyo Kasei Kogyo) was purified by distillation under reduced pressure. 1,3-Di(2-naphthyl)propane (1,3-22Np) and 1-(l-naphthyl)-3-(2-naphthyl)propane (1,3-12Np) were synthesized according to the method reported by Chandross and Dempster.I9 1,5-Di(2-naphthyl)pentane (1,5-22Np), 1,12di(2-naphthy1)dodecane (1,12-22Np), and rac- and meso-2,4di(2-naphthy1)pentane (2,4-rNp and 2,4-mNp, respectively) were prepared as previously r e p ~ r t e d .The ~ two diastereoisomers of 2,4-rNp and 2,4-mNp were separated by liquid chromatography (JASCO, TRI-ROTAR-V) fitted with a silica-gel column (Megapak SIL) with hexaneethyl acetate (1OOO:l v/v) as the eluent. The purity of both isomers was found to be 99% by liquid chromatography. 1,2-Dicyanobenzene (DCNB, Wako) was purified by recrystallization three times and triethylamine (TEA, Wako) was purified by distillation two times. Both chemicals were used as quenchers of Np. Acetonitrile (MeCN, Wako), the solvent for laser photolysis measurements, was refluxed over P 2 0 5several times and was fractionally distilled. 1-Chlorobutane, 2-methylbutane, and 2-methyltetrahydrofuran for T r a y irradiation were dried by the use of molecular sieves and then distilled prior to use. Spectroscopic Measuremertts. All samples for absorption, emission, and lifetime measurements were degassed mmHg) by repeated freezepumpthaw cycles in a Pyrex ampule fitted with a 1-cm quartz cell. The absorption spectra were measured by a UV-ZOOS (Shimadzu) spectrophotometer. The emission spectra were obtained by use of an 850 (Hitachi) fluorescence spectrophotometer. The lifetime of the emission was (16)(a) Shida, T.; Iwata, S . J . Am. Chem. Soc. 1973,95,3473. (b) Shida, T. J . Phys. Chem. 1978,82,991. (17) (a) Kira, A.; Arai, S.; Imamura, M. J . Phys. Chem. 1972,76, 1 1 19. (b) Kira, A.; Imamura, M.; Shida, T. J . Phys. Chem. 1976,80, 1445. (c) Kira, A,; Imamura, M. J . Phys. Chem. 1979,83, 2267. (18)El-Shall, M. S.;Meot-Ner (Mautner), M. J . Phys. Chem. 1987,91, 1088. (19)Chandross, E.A.; Dempster, C. J. J . Am. Chem. Soc. 1970,92,3586.
gd 0,:
4
0
ENFL : 0 675
0
Loo
500
6M)
700
800
Wavelength I nm Figure 2. (a) Transient absorption spectra of Np (8.1 X lo-) M) with DCNB (1.0 X lo-' M) obtained at 100 ns (open circles) and 500 ns (closed circles) after excitation in MeCN solvent at 298 K. (b) AbM) formed by y-ray sorption spectra of ion radicals of Np (5.0 X irradiation at 77 K: cation radical formed in a l-chlorobutane-2methylbutane (1:l v/v) mixed solvent after irradiation (solid line), after slight warming and returning to 77 K (broken line), and anion radical formed in 2-methyltetrahydrofuran solvent after irradiation (dotted line). (c) Transient absorption spectra of ENp (3.1 X lr3M) with DCNB (1.0 X lo-' M) obtained at 100 ns (open circles) and 500 ns (closed circles) after excitation in MeCN at 298 K. measured by the single-photon-countingmethod (PRA Inc., Model 510B). The transient absorption was obtained by use of a nanosecond laser photolysis apparatus as described elsewhere.20 Photoexcitation was made by a Lambda Physik EMGlOlE excimer laser, whose 308-nm light pulse has a 17-11s pulse duration. For detection a photomultiplier (Hamamatsu, R928) and an oscilloscope (Iwatsu, TS-8123) were used. Measurements were made at 298 K. The absorbance of all samples at 308 nm was adjusted to be nearly unity. The photoexcitation was selectively made on an N p group. Pyrex filters of an appropriate thickness were used to reduce the 308-nm exciting light pulse energy to avoid the effects of multiphoton excitation. DCNB (1.0 X lo-' M) and TEA (1.0 X lo-' M) were used as electron acceptor and donor for the N p group, respectively. The reference spectrum of the N p cation radical (Np'+) was measured by y-ray irradiation from a source (2 X lo6 rad h-l) in a glass matrix of 1-chlorobutane2-methylbutane(1:l v/v) mixture at 77 K. The absorption spectrum of the N p anion radical (Np'-) was obtained in a glass matrix of 2-methyltetrahydrofuran at 77 K. The measurements were made with a degassed 1-mm quartz cell.14
Results and Discussion Assignment of Transient Species. The electron-transfer quenching of the photoexcited singlet N p ('Np*) by the electron acceptor DCNB in a polar solvent of MeCN produces Np'+ and (20)(a) Tsuchida, A.; Yamamoto, M.; Nishijima, Y. J. Phys. Chem. 1984, 88, 5062. (b) Tsuchida, A.; Yamamoto, M.; Nishijima, Y. J. Chem. Soc., Perkin Trans. 2 1987, 507.
1246 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Tsuchida et al.
1,3-22Np --
1
0.5
0 120
aJ
0
C 0
f 0.5 0 cn
n
a 100ns l d i v Figure 3. Oscillograms of the transient absorption of Np (the same sample and conditions as shown in Figure 2a) at 570 nm (a) and 685 nm
0
(b). DCNB*-:21 INp* (8.1 X M) had a 91-11s lifetime in MeCN at 298 K, and the quenching rate constant of INp* by DCNB was determined by Stern-Volmer plots as 1.8 X l o i o M-' s-l, which corresponds to a diffusion-controlled value. Addition of 1.O X lo-' M DCNB reduced the fluorescence lifetime of INp* to less than 1 ns. Figure 2a shows the transient absorption spectra obtained by the laser photolysis measurements at 100 and 500 ns after excitation. N p (8.1 X M) was photoexcited by a 308-nm laser pulse in the presence of DCNB (1 .O X lo-' M) in an MeCN solvent at 298 K. In the spectra, four absorption bands at 685, 625,570, and 410 nm were recognized. To assign these absorption bands, the reference spectra of the N p ion radicals were measured by the y-ray irradiation technique in a glass matrix of l-chlorobutane-2-methylbutane (1:l v/v) mixture at 77 K. Figure 2b shows the spectra. The solid line shows the spectrum of Np": Two peaks at 700 and 640 nm for Np" correspond to the peaks at 685 and 625 nm in Figure 2a, respe~tively.'~,~~ The 15-nm shift is probably due to the difference in experimental temperature. On warming the glass matrix slightly from 77 K, the spectrum of Np" changed its shape as shown by the broken line in Figure 2b. The absorption band at 570 nm is ascribed to the dimer cation radical of N p (Np2*+).12*15-17922 The 570-nm band was also observed in the transient absorption spectrum shown in Figure 2a. Figure 3 shows the oscillograms of the transient absorption by laser photolysis. The oscillograms at 570 nm (a) and at 685 nm (b) were taken at the absorption maxima of Np2'+ and Np", respectively. These two oscillograms show almost the same rise and decay curves. The similarity in the pattern of the fast rise curves means that both the electron-transfer quenching process of INp* by DCNB and the dimer cation radical formation from Np'+ and a neutral N p are too fast to be resolved with our laser photolysis apparatus (time resolution, 4-5 ns; pulse fwhm, 17 ns). The similarity in the pattern of the decay curves of the absorption shows that the monomer cation radical (Np'+) is in equilibrium with Np2'+. Kira et al. examined the N p dimer cation radical formation by pulse radiolysis meas~rements.'~They determined the equilibrium constant K for Np2*+formation by changing the concentration of Np. The value K i n benzonitrile solution at 290 K was reported to be 5.2 X lo2 M-I, which corresponds to 3.6 kcal/mol exothermic free energy change for the dimer cation formation. This large value of K gives the large peak of Np2*+ (570 nm) in Figure 2a. Np2'+ also has an absorption band in the near-IR region. This band is called the charge-resonance (CR) (21) Grellmann, K. H.; Watkins, A. R.; Weller, A. J. Phys. Chem. 1972, 76, 469. (22) (a) Lewis, I. C.; Singer, L. S.; J . Chem. Phys. 1965, 43, 2712. (b) Rogers, M. A. J. Chem. Phys. Lett. 1971, 9, 107. (c) Biihler, R. E.; Funk, W. J . Phys. Chem. 1975, 79,2098. (d) Chandra, A. K.; Bhanuprakash, K.; Jyoti Bhasu, V. C.; Srikanthan, D. Mol. Phys. 1984, 52, 733.
0.5
0
LOO
500
600
700
800
Wavelength I nm Figure 4. Transient absorption spectra of dinaphthylalkanes (1.4 X M) with DCNB (1.0 X lo-' M) obtained at 100 ns (open circles) and 900 ns (closed circles) after excitation in MeCN solvent at 298 K: (a) 1,3-22Np,(b) 2,4-rNp, and (c) 2,4-mNp.
band,lsv17,22 the tail of which can be seen in parts a and b of Figure 2. The sharp absorption band at 410 nm in Figure 2a had a longer lifetime than the ionic species and is ascribed to the triplet state of Np (3Np*).13In the absence of DCNB, the transient absorption band of jNp* was observed at 410 nm with no absorption of ionic species. The anion radical of DCNB (DCNB-) has an absorption peak at 373 nm and the absorption in the visible region is negligible.20 Figure 2c shows the transient absorption spectra of the E N p DCNB (1.0 X 10-1M) system. The quenching rate constant of ENp by DCNB was determined by the Stern-Volmer plots to be 2.2 X 1OloM-I s-I. ENp is probably more suitable than N p for the monomer model of intramolecular dinaphthyl compounds. The absorption peaks at 675 and 620 nm are ascribed to the monomer cation of ENp (ENp"), though they are slightly blue-shifted from those of Np". In Figure 2c, the dimer cation radical band of ENp (ENp2'+) at around 570 nm as well as the CR band in the near-IR region are considerably small compared with those of Np2'+. The lower concentration of ENp (3.1 X M) due to the higher molar extinction coefficient of ENp at 308 nm and the steric hindrance of the ethyl group are considered to be responsible for the weak absorbance of ENp2'+. However, in the pulse radiolysis measurements of ENp in methylene chloride,12 a larger absorption peak of ENp2'+ than that of ENp'+ was observed in contrast to this experiment. The difference may be due to the difference in solvent polarity. Figure 4 shows the transient absorption spectra for the 1,32 2 N p D C N B , 2,4-rNp-DCNB, and 2,4-mNp-DCNB systems. The exciting laser power and the ground-state absorbances of samples at 308 nm were the same for all the samples and hence the height in the transient absorption spectra could be compared. Figure 4 shows that the transient absorption spectra for 1,3-22Np, 2,4-rNp, and 2,4-mNp are similar though slight changes in the peak positions and of the absorbances are recognized. In these spectra, three major absorption bands at ca. 660, ca. 580, and 420 nm with the near-IR band tail are observed. The band at 660 nm is different from the monomeric band of ENp'+, which is at
Conformations of Naphthalene Dimer Cation Radicals
1
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1247
I
1,3- 22Np (a'
1,3-12Np --
L20
(a'
I
(Cl
1,12-22N~
t
n "
I
LOO
500
600
700
800
Wavelength I nm Figure 5. Transient absorption spectra of dinaphthylalkanes with DCNB (1.0 X lo-' M) obtained at 100 ns (open circles) and 900 ns (closed
circles) after excitation in MeCN solvent at 298 K: (a) 1,3-12Np(1.0 X lo-' M), (b) 1,5-22Np(1.8 X IO-' M), and (c) 1,12-22Np(1.8 X IO-' M). 675 nm and shows a sharp peak (Figure 2c). This 660-nm band is inherent to 1,3-22Np, 2,4-rNp, and 2,4-mNp. The band at 580 nm is assigned to the dimer cation radical (Np2*+),12which was formed by the intramolecular interaction of two naphthyl groups. The intramolecular formation of Np2" was confirmed by the lack of the absorption of Np", which was produced by the equilibrium with Np2'+ as shown in Figure 2. The predominance of the intramolecular process and the lower concentration of naphthyl M) are considered to make the intercompounds (1.4 X molecular process only a minor process. The control experiment for the sample which has no DCNB showed that the band at 420 nm is attributed to 3Np*. Figure 5 shows the transient absorption spectra for the 1,312Np-DCNB, 1,5-22Np-DCNB, and 1,12-22Np-DCNB systems. Inspection of these spectra gives further detail on the Np;+ formation. Especially, the spectrum for 1,3-12Np (Figure 5a) provides important information on the Np2'+ structure. 1,3-12Np has two naphthyl groups, each of which is substituted at different positions, and this molecular structure does not allow a total overlap sandwich form (eclipsed form) of two naphthyl groups."J9 Only the staggered sandwich form, Le., partial overlap of naphthyl groups, is possible. This 1,3-12Np shows a band at 585 nm with the CR band tail in the near-IR region. The band at 420 nm is the absorption of 3Np*. The above consideration shows that the band at 585 nm is assigned to the staggered dimer cation radical of Np. The band at 585 nm is observed at 570-585 nm in Figures 2 and 4. It is interesting that the intermolecular dimer cation radical of N p or ENp has a staggered form. The Np2'+ formed by softening the rigid matrix after y-ray irradiation also has the same staggered form. On the other hand, 1,3-22Np, 2,4-rNp, and 2,4-mNp can take the total sandwich eclipsed form as well as the staggered form. Therefore, the eclipsed Np2'+ is the most probable candidate for the band at 660 nm in Figure 4. As for 1,5-22Np and 1,12-22Np, the absorption band of eclipsed Np2'+ at around 660 nm cannot be recognized as shown in Figure 5 , b and c. The band at 675 nm is ascribed to the monomer cation
"
11
LOO
1
-750
500
600
700
800
Wavelength I nm Figure 6. Transient absorption spectra of dinaphthylalkanes (1.4 X M) with TEA (1 .O X lo-' M) obtained at 50 ns (open circles) and 350 ns (closed circles) after excitation in MeCN solvent at 298 K: (a)
1,3-22Np,(b) 2,4-rNp, and (c) 2,4-mNp.
radical (Np'+). Slight absorption of staggered Np2*+at around 580 nm with the tail of the C R band can be seen in Figure 5b. 1,12-22Np shows a rather broad and red-shifted absorption band of Np2*+at around 600 nm. The staggered Np2'+ for these compounds is an intramolecular one as the concentration of dinaphthyls is low enough and the spectra are the same as other intramolecular systems. Probably the intramolecular staggered Np2'+ is in equilibrium with the monomer Np'+. Comparison of part a of Figure 5 with part c shows that Np2'+ formation is more favorable for 1,12-22Np than for 1,5-22Np. Zachariasse et al. reported a similar finding for the intramolecular excimer formation of pyrene (Py)chromophores.' The ratio of the excimer fluorescence to the monomer fluorescence they obtained for a series of intramolecular dipyrenyl compounds Py(CH2),Py showed a minimum at n = 7. The chain-length dependence of the intramolecular Np2*+formation strongly resembles the intramolecular excimer formation of Py. Figure 6 shows the transient absorption spectra for the 1,322NpTEA, 2,4-rNpTEA, and 2,4-mNpTEA systems. In these systems, electron transfer from TEA to N p produces TEA'+ and Np'-. The quenching rate constants of TEA were determined by the Stern-Volmer plots in MeCN at 298 K. They were 7.1 X lo9 and 1.8 X lo9 M-' s-l for N p and ENp, respectively. The spectrum of the anion radical of N p (Np'-) measured by the y-ray irradiation in 2-methyltetrahydrofuran rigid matrix at 77 K is shown by the dotted line in Figure 2b. Warming of the glass matrix decreased just the absorbance and did not change the spectrum shape. In contrast to dimer cations, the scarcity of data on dimer anion radical formation of aromatic molecules has been a mystery. Dimer anion radicals should be as stable as dimer cation radicals because the same arguments for the stability of dimer cations can apply to the dimer anion^.'^^^^ Aromatic dimer anion radical formation is observed only under limited experimental condition^.^^^^^ Absorption spectra for the three systems (23) (a) Arai, S.; Kira, A,; Imamura, M. J . Phys. Chem. 1977, 81, 110. (b) Shida, T.; Iwata, S. J . Chem. Phys. 1972,56, 2858.
1248 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
eclipsed
I
Sandwich Form eclipsed
1
staggered
Second Form
1
staggered
I
I
Figure 7. Sandwich and second form conformations of intramolecular bichromophoric compounds.
of dinaphthyl compounds are very similar and they look like the reference spectrum obtained by the y-ray irradiation. The C3 methylene chain is not sufficient for the dimer anion radical formation. Conformations of Dimer Cation Radicals. Figure 7 shows the sandwich and second form conformations of 1,3-di(Ncarbazolyl)propane, 1,3-22Np, and 43-1 2Np. This classification is based on the sandwich and second conformations of 1,3-di(N-carbazolyl)propane, which are shown by structures a-1 and a-2 in Figure 7 , respectively. Masuhara et al. measured the transient absorption spectra of 2,4-di(N-carbazolyl)pentanes. They showed that the sandwich dimer cation radical of carbazole (a-1) has an absorption band at around 770 nm, whereas that of the second dimer cation radical (a-2) appears at around 710 nmS6As for the N-carbazolyl chromophore, the sandwich dimer cation radical is formed only in the eclipsed form. On the other hand, the naphthyl compound 1,3-22Np can take both the eclipsed (b-1) and staggered (b-2) forms in the sandwich conformation, while 1,3-12Np, can take only staggered forms (c-1 and c-2) in the sandwich conformation. The dimer cation radical of N p in the staggered form has an absorption band at around 580 nm. The spectrum of intermolecular Npz'+ (570 nm in Figure 2) shows that the intermolecular Npz'+ has a staggered conformation. 1,5-22Np and 1,12-22Np, which have longer methylene chains than n = 3, also have the staggered dimer cation as shown in Figure 5 , b and c, respectively. These facts show that the favorable conformation for Npz'+ is the staggered form rather than the eclipsed one. According to MO calculation, the conformation of dimer cation radical is determined by the balance between the attractive valency forces and the repulsive forces of Pauli repulsion between filled orbitals. Badger and Brocklehurst proposed the distorted Npz*+as a stable conformation of the dimer cation r a d i ~ a l . ' ~In~ addition J ~ ~ to the electronic factors, steric factor also may govern the conformation. Intramolecular naphthyl compounds such as 1,3-22Np, 2,4-rNp, and 2,4-mNp have an absorption band at around 660 nm, which is ascribed to the absorption of the eclipsed dimer cation radical of N p as shown in structure b-1 of Figure 7. The linkage of two chromophores by the C3 methylene chain offers easy formation of the sandwich conformation of the chromophore^.^ As for the second dimer cation radical of N p chromophore, no evidence has been obtained so far. Structures b-3 and c-3 of Figure 7 show the second-form conformations of 1,3-22Np and 1,3-12Np, respectively. Probably, the rather remote distance of the two naphthyl chromophores in the second form compared with the distance of the two carbazolyl chromophores (Figure 7 , structure a-2) prohibits the formation of a second dimer cation radical. Process of the Formation of Dimer Cation Radicals. The ground-state conformations of the two diastereoisomers of 2,4di(N-carbazoly1)pentane were determined by De Schryver et aL4 (24) Tsuchida, A,; Masuda, N.; Yamamoto, M.; Nishijima, Y. Macromolecules 1986, 19, 1299.
Tsuchida et al. The analysis of IH NMR spectra revealed that the racemic isomer takes 82% TT and 18% G G conformations at 313 K in CD2C12. For the meso isomer at room temperature, the TG/GT conformation is most stable. The distribution of the ground-state conformation changes only a little depending on the temperature, solvent, and chromophore. However, their finding is applicable to the case of intramolecular dinaphthyl compounds. In the laser photolysis measurements, the transient ionic species are produced by the photoinduced electron transfer process from an electron donor to acceptor in a polar solvent. The electrontransfer quenching of N p by DCNB in MeCN at 298 K was of a diffusion-controlled process as described above. Therefore, the Np" and DCNB'- are produced within 1 ns after the photoexcitation in the presence of DCNB (1.0 X lo-' M). As for intramolecular dinaphthyl compounds such as 1,3-22Np, 2,4-rNp, and 2,4-mNp, which satisfy the Hirayama's n = 3 rule,3 intramolecular excimer of Np is formed efficiently. The rate constants of intramolecular excimer formation of 1,3-22Np, 2,4-rNp, and 2,4-mNp were 21 X lo7, 8.7 X IO', and 79 X lo7 s-] at 298 K in tetrahydrofuran solvent, re~pectively.~2,4-mNp shows the fastest excimer formation and the process may compete with the electron-transfer process. However, the excimer thus produced was also quenched by DCNB to produce Npz*+and DCNW-, the quenching process of which was observed from their emission spectra. As shown in Figure 4, 1,3-22Np, 2,4-rNp, and 2,4-mNp gave similar transient absorption spectra of ionic species. This shows that the differences in the ground-state conformation distribution and of the excimer formation rate do not affect the conformation distribution of the produced dimer cation radicals. In this study, it is concluded that the dimer cation radicals of intramolecular dinaphthyl compounds are produced in the eclipsed and/or staggered forms of the sandwich conformation. Whether Np2'+ is eclipsed or staggered is determined by the direction of the N p group. The dimer cation radical in the second form could not be found. In the case of 2,4-di(N-carbazolyl)pentanes, the second dimer cation radical is produced from the T T conformation of the racemic isomer by a small conformational change, and the process is very fast, while the sandwich dimer cation radical is produced from the TG/GT conformation of the meso isomer. The latter process requires the rotation around a main chain C-C bond of the methylene chain. As for the racemic isomer of the dinaphthyl compound (2,4-rNp) which cannot make a second excimer, the excimer formation has been shown to require a longer time and more activation energy than 1,3-22Np or 2,4-mNp.'v9 The formation of Npz*+ is considered to need the same conformational change as in the excimer formation, and the order of the forming rate is also expected to be the same. The laser pulse width (17 ns) of our laser photolysis apparatus seems somewhat insufficient to observe directly the process of the formation of Npz'+. The ratio of two types of Npz*+for 1,3-22Np, 2,4-rNp, and 2,4-mNp are similar; the rotational direction of the naphthyl group, which determines the fraction of the eclipsed and staggered Npz'+, seems random for the Np,'+ forming process for 1,3-22Np, 2,4-rNp, and 2,4-mNp. The dimer cation radical formation process from 3Np* was negligible in this study. The quenching rate of 3Np* by DCNB was small and hardly any the electron transfer from 3Np* to DCNB occurred in the time scale of this measurement. Acknowledgment. We thank Professor Hitoshi Yamaoka of the Research Reactor Institute, Kyoto University, for his help in determining the reference absorption spectra of the ion radicals. We also thank Professor Akira Kira of the Institute of Physical and Chemical Research for valuable comments and suggestions. Registry No. Np'', 34512-27-1; ENp", 75281-56-0; 1,3-22Np'+, 75240-59-4; 1,5-22Np'+, 117775-96-9; 1,12-22Np'+, 117775-97-0; 1,312Np'+, 29571-16-2;2,4-rNp'+, 117859-30-0;2,4-mNp'+, 117893-44-4.
